Intra and Inter-Observer Reliability and Repeatability of Metatarsus Adductus Angle in Recreational Football Players: A Concordance Study

Metatarsus adductus (MA) is a congenital foot deformity often unrecognized at birth. There is adduction of the metatarsals, supination of the subtalar joint, and plantarflexion of the first ray. The aims of this study were to assess the intra and inter-reader reliability of the radiographic MA measurement angles used in the literature. Methods: All consecutive recreational football players who practice activity more than 5 h/week over 21 years of age with MA by roentgenographic evaluation on weight-bearing dorsoplantar images were included in a cross-sectional study. Controls were matched to cases according to age and gender. We assess all radiographic measurements to evaluate metatarsus adductus with the different measurements frequently used in the literature: Sgarlato, modified Sgarlato, Rearfoot, Root, Engel, modified Engel, Kite, Kilmartin, modified Kilmartin, Simons, and Laaveg & Ponseti. Results: The variables measured in 80 weight-bearing dorsoplantar foot radiographs show excellent reliability ranging p > 0.900 in Sgarlato and modified Sgarlato with low SEM, CV, and MCD. Rearfoot, Root, Engel, modified Engel, Kite, Kilmartin, Simons, Laaveg & Ponseti, and modified Kilmartin’s angles showed intra or inter reliability with ICC lower than <0.900, systematic differences between intersession or inter observers, or high MCD value. Conclusion: It is more suitable to measure the MA angle with the Sgarlato and modified Sgarlato techniques to show higher reliability and repeatability for intra and inter-observer.


Introduction
Metatarsus adductus (MA) is a relatively common congenital foot deformity that is often unrecognized at birth [1]. There is adduction of the metatarsals, supination of the subtalar joint, and plantarflexion of the first ray [2]. Metatarsus adductus is defined as a transverse plane deformity in which the metatarsals deviate medially in relation to the midfoot. This can present as one of the deformities associated with clubfeet in the paediatric population, or be observed in adolescents and adults [3][4][5][6]. The frequency of occurrence of metatarsus adductus varies from 0.1% to 12% [7][8][9]. This pathology is not completely clear [10,11]. The exact aetiology of metatarsus adductus is unknown, though it has been suggested that increased intrauterine pressure, osseous abnormality, and abnormal muscle attachments may be causes [12].
A growing body of evidence has investigated the reliability of various angular and linear measurements on X-rays of the foot and ankle. The reliability of these measurements

Sample Size Calculation
The required sample size was calculated based on reliability testing to determine reliability. In this study, the ICCs were used for reliability testing at a target value of 0.8 and a 95% CI of 0.2. We calculated the sample size to be 36 patients with Bonett's approximation [19].

Ethical Aspects
This research was approved by the local Research Ethical Committee at the University of Rey Juan Carlos (URJC) in Madrid, with internal register number 0212201600117. The required local regulations and ethical standards for human experimentation of the Declaration of Helsinki were respected [20].

Subjects
The sample was recreational football players who practice more than 5 h/week, attending the foot and ankle clinical unit at the CEMTRO hospital of Madrid from September 2019 to February 2020. All consecutive recreational football players over 21 years, to confirm the closure of the epiphyseal plate [21], with MA by roentgenographic evaluation, were included in a cross-sectional study. The control group included recreational football players without MA who visited the clinic for other orthopedic conditions of the foot. Controls were matched to cases according to age and gender.
Foot images were taken using a General Electric Discovery XR656 Plus (General Electric Research, Milwaukee, WI) at a source-to-image distance of 100 cm and were set to 60 kVp and 2.5 mAs. We retrieved the radiographic images using a picture archiving and communication system (PACS) (IMPAX; Agfa Healthcare, Mortsel, Belgium), and radiographic measurements were performed using PACS software and a digital radiographic imaging and measuring system (AutoCad 2019, Autodesk Inc., San Rafael, CA, USA). Dorsoplantar radiographs for weight-bearing conditions were performed independently for each foot with the patients standing with the knee extended. The medial border of the foot was aligned to avoid internal or external rotation of the leg and the foot was pointed straight forward in neutral rotation, parallel to the medial sagittal plane [22]. The X-ray beam was inclined 15 • in an anterior-posterior direction centered on the second tarsometatarsal joint at a distance of 100 cm [23][24][25][26][27].
We reviewed 143 weight-bearing dorsoplantar foot radiographs and excluded 63 of them. Inclusion criteria were no evidence of foot trauma that could affect foot anatomy and no history of previous forefoot, midfoot, or rearfoot surgery. Exclusion criteria were being a pregnant woman, having a neurologic disease, lower limb malformation, fracture, or previous surgery of the foot or lower limb, or practicing football less than 5 h/week. We establish two criteria groups: the MA group > 20 • [10,28,29] and the control group ≤ 20 • . Measurements included 80 radiographs from 40 feet of men (20 in the control group and 20 in the MA group) and 40 feet of women (20 in the control group and 20 in the MA group).
The following angles were examined on each X-ray: Sgarlato's angle [6,30,31], Figure 1, is the angle between the longitudinal axis of the 2nd metatarsal and the longitudinal axis of the lesser tarsus using the 4th metatarso-cuboid joint. Line (a) extends between the most medial point of the talo-navicular and the medial cuneiform-first metatarsal joints, while line (b) extends between the most lateral point of the 4th metatarso-cuboid and the calcaneo-cuboid joints. Line (c) extends between the midpoints of lines (a) and (b). Line (d) is perpendicular to line (c) and represents the longitudinal axis of the lesser tarsus. Line (e) represents the longitudinal axis of the second metatarsal bone. Sgarlato's angle is between lines (d) and (e). and no history of previous forefoot, midfoot, or rearfoot surgery. Exclusion crite being a pregnant woman, having a neurologic disease, lower limb malformation, f or previous surgery of the foot or lower limb, or practicing football less than 5 h/w establish two criteria groups: the MA group > 20° [10,28,29] and the control grou Measurements included 80 radiographs from 40 feet of men (20 in the control gr 20 in the MA group) and 40 feet of women (20 in the control group and 20 in group).
The following angles were examined on each X-ray: Sgarlato's angle [6,30,31 1, is the angle between the longitudinal axis of the 2nd metatarsal and the long axis of the lesser tarsus using the 4th metatarso-cuboid joint. Line (a) extends betw most medial point of the talo-navicular and the medial cuneiform-first metatarsa while line (b) extends between the most lateral point of the 4th metatarso-cuboid calcaneo-cuboid joints. Line (c) extends between the midpoints of lines (a) and ( (d) is perpendicular to line (c) and represents the longitudinal axis of the lesse Line (e) represents the longitudinal axis of the second metatarsal bone. Sgarlato's between lines (d) and (e). Modified Sgarlato's [18,[30][31][32], Figure 2, is the angle between the longitudina the second metatarsal and the longitudinal axis of the lesser tarsus using metatarso-cuboid joint as a reference. Line (a) extends between the most lateral Modified Sgarlato's [18,[30][31][32], Figure 2, is the angle between the longitudinal axis of the second metatarsal and the longitudinal axis of the lesser tarsus using the 5th metatarsocuboid joint as a reference. Line (a) extends between the most lateral point of the 5th metatarso-cuboid and the calcaneo-cuboid joints. Line (b) extends between the most medial point of the talo-navicular and the medial cuneiform-first metatarsal joints. Line (c) extends between the midpoints of lines (a) and (b). Line (d) represents the longitudinal axis of the second metatarsal bone. Line (e) is perpendicular to line (c) and represents the longitudinal axis of the lesser tarsus. Sgarlato's angle is between lines (d) and (e). the 5th metatarso-cuboid and the calcaneo-cuboid joints. Line (b) extends be most medial point of the talo-navicular and the medial cuneiform-first metatar Line (c) extends between the midpoints of lines (a) and (b). Line (d) repre longitudinal axis of the second metatarsal bone. Line (e) is perpendicular to li represents the longitudinal axis of the lesser tarsus. Sgarlato's angle is between and (e). The Rearfoot angle [30,33], Figure 3, is the angle between a parallel line to border of the calcaneum and second metatarsal axis. Line (a) is parallel to t border of the calcaneum, line (b) is parallel to line (a), line (c) axis 2nd metata Rearfoot angle is between line (b) and line (c). Root's angle [5], Figure 4, measures the adduction of the forefoot by using the 5th metatarso-cuboid and the calcaneo-cuboid joints. Line (b) extends be most medial point of the talo-navicular and the medial cuneiform-first metata Line (c) extends between the midpoints of lines (a) and (b). Line (d) repr longitudinal axis of the second metatarsal bone. Line (e) is perpendicular to li represents the longitudinal axis of the lesser tarsus. Sgarlato's angle is betwee and (e). The Rearfoot angle [30,33], Figure 3, is the angle between a parallel line to border of the calcaneum and second metatarsal axis. Line (a) is parallel to border of the calcaneum, line (b) is parallel to line (a), line (c) axis 2nd metat Rearfoot angle is between line (b) and line (c). Root's angle [5], Figure 4, measures the adduction of the forefoot by using formed between the longitudinal axis of the 2nd metatarsus, line (b), and the lon axis of the rearfoot, line (a).  Engel's angle [34], Figure 5, is the angle between the longitudinal axis of t cuneiform as shown at line (b) and the longitudinal axis of the 2nd metatarsa shown at line (a). Modified Engel's angle [35], Figure 6, is the angle between the longitudin the 2nd metatarsal, line (c), and a line perpendicular to the proximal articular  Engel's angle [34], Figure 5, is the angle between the longitudinal axis of cuneiform as shown at line (b) and the longitudinal axis of the 2nd metatars shown at line (a).  Kilmartin's angle [36], Figure 8, is measured between a line parallel to the late border of the calcaneum and the 1st metatarsus axis. Line (a) is parallel to the late   Modified Kilmartin's angle [14], Figure 9, is the angle measured between the longitudinal axis of the second metatarsal and the transverse axis of the lesser tarsus. Line (a) extends between the most medial point of the talo-navicular and the medial cuneiformfirst metatarsal joints. Line (b) extends between the most lateral point of the 4th metatarsocuboid and the calcaneo-cuboid joints. Line (c) extends between the midpoints of lines (a) and (b). Line (d) represents the longitudinal axis of the second metatarsal bone. Modified Kilmartin's angle is the angle between lines (c) and (d).  Modified Kilmartin's angle [14], Figure 9, is the angle measured between the longitudinal axis of the second metatarsal and the transverse axis of the lesser tarsus. Line (a) extends between the most medial point of the talo-navicular and the medial cuneiformfirst metatarsal joints. Line (b) extends between the most lateral point of the 4th metatarsocuboid and the calcaneo-cuboid joints. Line (c) extends between the midpoints of lines (a) and (b). Line (d) represents the longitudinal axis of the second metatarsal bone. Modified Kilmartin's angle is the angle between lines (c) and (d).

Reliability Study Protocol
Intra and inter-rater reliability study: All radiographs were selected for inclusion in the intra-rater reliability study. Two evaluators conducted all measurements for each radiograph at three different times, separated by 1 week each. In total, 6 measurements in each of the different angles of the Rx were taken.
Before each measurement session, the order of the patients was randomized so that the radiographs viewed would not be in the same order as that of the previous week's measurements.
To guarantee the quality of the data and provide a benchmark for the reproducibility of each measurement, the two readers independently measured each angle on a radiograph in two different cities. The readers were physicians with more than 15 years Laaveg & Ponseti's angle [38], Figure 11, is the angle between a line parallel to the lateral surface of calcaneum bone, line (a), and the longitudinal axis of the 5th metatarsal bone, line (b).

Reliability Study Protocol
Intra and inter-rater reliability study: All radiographs were selected for inclusion in the intra-rater reliability study. Two evaluators conducted all measurements for each radiograph at three different times, separated by 1 week each. In total, 6 measurements in each of the different angles of the Rx were taken.
Before each measurement session, the order of the patients was randomized so that the radiographs viewed would not be in the same order as that of the previous week's measurements.
To guarantee the quality of the data and provide a benchmark for the reproducibility of each measurement, the two readers independently measured each angle on a radiograph in two different cities. The readers were physicians with more than 15 years

Reliability Study Protocol
Intra and inter-rater reliability study: All radiographs were selected for inclusion in the intra-rater reliability study. Two evaluators conducted all measurements for each radiograph at three different times, separated by 1 week each. In total, 6 measurements in each of the different angles of the Rx were taken.
Before each measurement session, the order of the patients was randomized so that the radiographs viewed would not be in the same order as that of the previous week's measurements.
To guarantee the quality of the data and provide a benchmark for the reproducibility of each measurement, the two readers independently measured each angle on a radiograph in two different cities. The readers were physicians with more than 15 years of radiology experience and were trained on an initial set of three cases from the sample under the direction of another senior physician with more than 30 years of radiology experience. All measurements were recorded on a Microsoft Excel spreadsheet.

Statistical Analysis
Quantitative data were described as mean ± standard deviation (SD) as well as lower and upper limits for a 95% CI, and median and interquartile range All variables were examined for normality of distribution using the Kolmogorov-Smirnov test; data were considered normally distributed if p > 0.05. Independent Student t-tests were performed to determine whether differences were statistically significant when a normal distribution was shown. Measurements that were not normally distributed were tested using a non-parametric U Mann Whitney test.
Reliability between two measurement values was determined using the Intraclass Correlation Coefficient two-way random effects, absolute agreement, multiple raters/measurements (ICC 2,k), and Pearson s correlation coefficient (r). The average of two trials for each test session on each radiograph was used to calculate intersession reliability. ICC values were interpreted as poor (ICC < 0.40), fair (ICC = 0.40-0.59), good (ICC = 0.60-0.74), and excellent (ICC = 0.75-1.0) [39]. According to the recommendations of Portney and Watkins, clinical measurements with reliability coefficients greater than 0.90 improve the probability that the measure is valid [40].
Coefficients of variation (CVs) were analyzed for the absolute comparison of parameters. The CVs were calculated to test the intra-session reliability. The CV was calculated as the mean normalized to the SD. A higher CV value shows that the data are more heterogeneous.
The 95% limits of agreement (LoA) between sessions and devices expressed the degree of error proportional to the mean of the measurement units, and these statistics were calculated using the methods described by Bland and Altman [42]. If the differences between the measurements tended to agree, the results were close to zero.
Standard errors of measurement (SEM) were calculated to measure the range of error of each parameter. SEM were calculated from the ICCs and SDs for each of the three measurements, according to the formula SEM = SD × sqrt (1-ICC).
The minimum detectable change (MDC) at a confidence level of 95%, which reflects the magnitude of change necessary to provide confidence that a change is not the result of random variation or measurement error, was calculated from the SEM values by the following formula: MDC = √ 2 × 1.96 × SEM. Both SEM and MDC were analyzed according to Bland and Altman [42].
In all of the analyses, statistical significance was established by a p value of less than 0.05, with an interval of confidence of 95%, and analyses were performed with commercially available software (SPSS 25.0, SPSS Inc., Chicago, IL, USA).

Results
In total, 80 participants were recruited. Descriptive data of the age variable showed a normal distribution (p = 0.216), as shown in Table 1.   Table 3  SEM was low for all variables except Kite's angle, SEM = 7.876, MCD on modified Engel's angle was 7.199, while that for the rest of variables was low. CV was low for all variables. We found systematic differences between sessions on Rearfoot's angle and Kilmartin's angle (p < 0.05). Table 4 shows an analysis of the reliability of metatarsus adductus angle measurements between observers. Excellent ICC values were obtained between observers ICC > 0. 2)], respectively. We found systematic differences inter-observer on kite's angle and modified Kilmartin's angle (p < 0.05).

Discussion
Lack of consensus among physicians could be due to the lack of agreement regarding classification. Metatarsus adductus measurement should be simple, easy to remember, and effective for all physicians. Various radiographic measurements have been developed and used for evaluating metatarsus adductus [5,14,30,32,33,43]. The objective of this study was to update guidelines regarding the inter-observer reliability of the common measurement most currently used in MA assessment [30,32,34,35]. Before discussing the findings of our study, we must indicate limitations. Metatarsus adductus is a complex three-dimensional deformity, and we only can explain with this study in 2-dimensional plane.
The intra and inter-observer reliability of common angular measurements of various foot disorders has been reported to be satisfactory [14]. The reliability of the angular measurements used to evaluate MA has been reported to a much lesser extent than other radiological foot measurements [44].
In each session, the first and second observers, and also the inter-observer, present high ICC the following measurements: Sgarlato and modified Sgarlato, both with ICC > 0.900; in addition, we found low SEM and CV. No systematic differences were found between sessions on these measurements. Sgarlato and modified Sgarlato's angles' ICC values are in concordance with the values shown by Dominguez [18] and Dawoodi [44].
On the other hand, the measurement angles for Root, modified Engel, Kite, and modified Kilmartin show an ICC < 0.900 in any session, intersession, or inter-observer and should be higher, as indicated by Portney [40].
Likewise, other angle measurements, like Engel, modified Engel, Rearfoot, modified Kilmartin, Kite, and Root, showed systematic differences in any intersession or interobserver measurements. Simon's angle presents a high MCD value between observers

Limitation of This Study
This study has been achieved in adults; thus, further research is needed to find intra and inter-observer MA in children. Furthermore, computed tomography (CT) scan is a useful diagnostic tool to produce a 3D image of bones; further research is needed to find the reliability and repeatability of metatarsus adductus angles as a new method, as postulated by Spinarelli A et al. [45].

Conclusions
Our study suggests that different techniques used in assessing metatarsus adductus demonstrated high values of intra and inter-observer reliability, Sgarlato and modified Sgarlato's angle like in other studies. On the other hand, we do not advise using the measures Root, Rearfoot, Engel, modified Engel, Kilmartin, modified Kilmartin, Laaveg & Ponseti, and Kite for presenting an ICC lower than 0.900 or systematic differences between intersession and/or inter-observers, and Simons for presenting a high MCD value. Informed Consent Statement: In addition, written consent was previously obtained for all participants.

Data Availability Statement:
The datasets used and/or analyzed in the current study or any query regarding to the research process are available from the corresponding author.